1. Field of the Invention
The present invention relates to a tapered roller. This tapered roller is applicable to, for example, a tapered roller bearing employed in a shaft supporting portion of a transmission of automobiles.
2. Description of the Related Art
In recent years, ATs and CVTs are increasingly used as automobile transmissions, and there is a tendency to use low viscosity oil in the transmissions in order to improve fuel efficiency. In the environment in which low viscosity oil is used, very short-life surface-originated flaking caused by poor lubrication may occur on the inner ring raceway surface receiving high contact pressure when the following adverse conditions are simultaneously present: (1) the temperature of the oil is high; (2) the amount of the oil is small; and (3) loss of preload occurs.
A direct and effective solution for the reduction in life due to the surface-originated flaking is to reduce maximum contact pressure. In order to reduce the maximum contact pressure, the dimensions of a bearing are changed, or the number of rollers of the bearing is increased when the dimensions of the bearing are not changed. In order to increase the number of rollers without reducing the diameter of the rollers, the distance between pockets of a retainer must be reduced. Therefore, the size of the pitch circle of the retainer must be increased to bring the retainer as close to an outer ring as possible.
FIG. 7 shows a tapered roller bearing (see Japanese Patent Laid-Open Publication No. 2003-28165) as an example in which a retainer is brought close to an outer ring until the retainer is brought into contact with the inner diameter surface of the outer ring. In this tapered roller bearing 61, the outer peripheral surface of a small-diameter-side annular portion 62a of a retainer 62 and the outer peripheral surface of a larger-diameter-side annular portion 62b of the retainer are brought into sliding contact with the inner diameter surface of an outer ring 63 to thereby guide the retainer 62. Furthermore, a recess 64 is formed in the outer diameter surface of a pillar portion 62c of the retainer 62 in order to suppress dragging torque, thereby maintaining a non-contact state between the outer diameter surface of the pillar portion 62c and a raceway surface 63a of the outer ring 63. The retainer 62 has the small-diameter-side annular portion 62a, the larger-diameter-side annular portion 62b, and a plurality of the pillar portions 62c each of which axially connects the small-diameter-side annular portion 62a and the larger-diameter-side annular portion 62b and has the recess 64 formed in the outer diameter surface thereof. Furthermore, a plurality of pockets for rollably receiving a tapered roller 65 is provided between the plurality of the pillar portions 62c. In the small-diameter-side annular portion 62a, a flange portion 62d integrally extending inward is provided. The tapered roller bearing shown in FIG. 7 is designed to improve the strength of the retainer 62. In addition, this tapered roller bearing is an example in which the retainer 62 is brought close to the outer ring 63 until the retainer 62 is brought into contact with the inner diameter surface of the outer ring 63 in order to increase the circumferential width of the pillar portion 62c of the retainer 62.
In recent years, automobile transmissions and other parts in which a roller bearing is employed are becoming increasingly small, and the output power thereof is more and more increased. Therefore, the viscosity of lubricating oil is reduced, and in the use environment of the lubricating oil, the load on and temperature of a roller bearing tend to increase. Therefore, the lubrication environment of bearings becomes more severe than ever, and wear and surface-originated flaking due to poor lubrication are much more likely to occur.
In each of Japanese Patent Laid-Open Publications Nos. Hei 2-168021 and Hei 6-042536, a roller bearing is described in which minute irregularities are formed on the surface of a rolling element to improve the ability to form an oil film. Such conventional indentations having a micro recess-like shape are formed such that, when the surface roughness is represented by a parameter Rqni, the value of the ratio of an axial surface roughness Rqni(L) to a circumferential surface roughness Rqni(C) (i.e., Rqni(L)/Rqni(C)) is 1.0 or lower (Rqni≧0.10), and that the value of a surface roughness parameter Sk is −1.6 or less. Hence, not only when a surface to be brought into contact with the rolling element is a rough surface but also when this surface is a well-finished surface, a long life is obtained. However, the thickness of an oil film is extremely small under the conditions of low viscosity-lean lubrication. In this case, the effects of the indentations may not be sufficiently obtained.
In the tapered roller bearing 61 described in Japanese Patent Laid-Open Publication No. 2003-28165, the retainer 62 is brought in the outward direction until the retainer 62 is brought into contact with the inner diameter surface of the outer ring 63 to thereby increase the circumferential width of the pillar portion 62c of the retainer 62. Furthermore, since the pillar portion 62c of the retainer 62 has the recess 64, the thickness thereof is necessarily decreased to cause the reduction of the stiffness of the retainer 62. Therefore, for example, stresses at the time of the assembly of the bearing 61 may cause deformation of the retainer 62, and the retainer 62 may be deformed during the rotation of the bearing 61. Meanwhile, in a conventional tapered roller bearing with a typical retainer other than the tapered roller bearing described in Patent Document 1, an outer ring 71 is prevented from being in contact with a retainer 72 as shown in FIG. 8. Furthermore, in order to ensure the pillar width of the retainer 72 and to obtain suitable pillar strength of the retainer 72 and smooth rotation, this tapered roller bearing is designed such that a roller coefficient γ (the filling factor of rollers) defined by the following equation is normally 0.94 or less.
Roller coefficient γ=(Z×DA)/(π×PCD). Here, Z is the number of rollers, DA is the average diameter of the rollers, and PCD is the diameter of the pitch circle of the rollers.
Furthermore, in FIG. 8, reference numeral 73 represents a tapered roller, and reference numeral 74 represents a pillar surface. Reference numeral 75 represents an inner ring, and symbol θ represents a window angle.